There exists a wide variety of power sources for powering electronic devices. For example, in the United States and Japan the standard AC voltage is 10V, while the AC standard voltage in Europe, Australia and other countries is 240V. When connecting an electronic device to a power source or any electric circuit, it may be beneficial to confirm the presence of the required voltage to the electronic device.
Presently, available low voltage detector circuits are used to detect the presence of a voltage from a power source. An opto-isolator is an electrical component typically used in a low voltage detector circuit to optically transfer a signal between an input and an output circuit, such as between a low voltage and a high voltage circuit. The opto-isolator helps to electromagnetically isolate the circuits from one another and from potentially destructive voltage spikes. Unlike a voltage transformer, an opto-isolator removes ground loops and excess noise or electromagnetic interference (EMI), and provides protection from serious over voltage conditions. Generally, a voltage detector circuit includes an opto-isolator to detect the presence of a voltage, and also includes a sensing resistor in series with the opto-isolator. The use of the sensing resistor may be undesirable in some applications because the resistor must handle an excessive power dissipation which leads to high impedance noise pulses. Unfortunately, a sensing resistor is expensive, and typically dissipates a substantial amount of heat.